Selective browsing behaviour of ungulates influences the growth of Abies alba differently depending on forest type

Abstract

Ungulate densities and browsing have increased over the past decades in many forests in Europe. Browsing on tree saplings is a selective process dependent on forest type. This study tested whether the impacts of browsing are altered by differences in tree vigour and within-tree browsing intensity (browsing only buds vs. browsing a large part of the annual leader shoot), and if these effects are modulated by forest type.The growth rate and within-tree browsing intensity of leader shoots were investigated for different height classes and species compositions at 18 sites (each with 14–64 plots) in spring before budburst and at 6 sites in autumn. The sites were situated in northeast Switzerland and comprised four major forest types that had Abies alba regeneration. Linear mixed-effects models were fitted for the relative growth rate of Abies and for the ratio of the relative growth rate of Abies to the relative growth rate of Picea abies.More Abies saplings were present in Fagus-dominated forests than in Fagus-Abies or Picea-Abies forests, and within-tree browsing intensity on their leader shoots was lower. Lightly browsed Abies saplings grew better than those that were not browsed, which in turn grew better than strongly browsed saplings. This pattern, which occurred irrespective of forest type, was caused by selective browsing on vigorously growing trees and led to a greater impact of strong browsing in comparison to light browsing on the growth of Abies saplings.The ratio of the relative growth rate of Abies to Picea was altered by within-tree browsing intensity, forest type and soil depth. Generally, this ratio was highest in shallow soiled Fagus-dominated forests after light browsing and lowest in Fagus-Abies forests after strong leader shoot browsing, indicating a browsing-induced shift in the relative difference in growth rate between species towards Picea in Fagus-Abies and Picea-Abies forests but not in Fagus-dominated forests.Because the main factor influencing the growth of Abies saplings was the amount of tissue loss on the leader shoots (bud vs. entire or large parts of leader shoots), browsing inventories neglecting to assess the within-tree browsing intensity are not recommended. The within-tree browsing intensity of leader shoots is a simple but objective measurement that should be used in forest regeneration inventories of Abies for improving estimates of the impact of ungulate browsing.